Mesh can't "keep up" with bone movement
Clicking on the 'windows' object, changing to Weight Paint mode and clicking on each of the vertex groups in turn we can see that the small triangle of window is part of the 'door_fl' group in addition to the 'car_body' group (red highlighting means a weight of '1' is assigned):



The small part of the window that is 'lagging behind' needs to be solely part of the 'door_fl' group. Because it is also assigned to the 'car_body' group (and therefore also under the control of the 'car_body' bone), the 'door_fl' bone is not fully in control, which is why it is not exactly matching the movement of the bone. When vertices have weights of '1' in two groups, the bones connected to those groups share control of the vertices 50/50. 

To remedy this, simply select the vertices that make up that part of the window in edit mode, click the 'car_body' group in the Vertex Group panel of the Object Data tab in the Properties editor and click the Remove button:



The window will now be only in the 'door_fl' group, only be controlled by the 'door_fl' bone and will move correctly.